That's because it assumes its users are intelligent. For example, it
also does not contain the word "writes". The reasoning is: it is
sufficient to include the word "write", and define it as a verb, since
all derivations therefrom are regular. It only lists derivations which
are irregular (e.g. "noticeable")

But both writable and writeable are legal in certain circles. Only one of them
can be considered regular.